Conventionally, there has been known a process for producing expanded particles by dispersing polyolefin resin particles together with a foaming agent into an aqueous dispersion medium, impregnating the resin particles with the foaming agent at a constant pressure and constant temperature after raising the temperature, and then releasing them into a low-pressure atmosphere. As for the foaming agent, disclosed examples of such processes include processes that involve the use of volatile organic foaming agents such as propane and butane (e.g., Patent Literature 1) and processes that involve the use of inorganic gasses such as carbon dioxide, nitrogen, and air (e.g., Patent Literatures 2 and 3).
However, the volatile organic foaming agents are substances that are greater in global warming potential than carbon dioxide, and as such, they are not environmentally preferable. Further, the volatile organic foaming agents, such as propane and butane, have a capacity to plasticize polyolefin resin and therefore make it easy to attain a high expansion ratio, but because their plasticizing capacity is great, they tend to make it difficult to control the expansion ratio and crystal condition of expanded particles. Further, the volatile organic foaming agents are flammable substances, and as such, they make it necessary to make facilities explosion-proof, thus incurring high costs in facilities.
Meanwhile, use of the inorganic gases, such as nitrogen and air, results in the incapability of, even under high pressure, attaining an amount of impregnation sufficient for a higher level of expansion, due to their very low capacity to spread completely through polyolefin resin.
In order to overcome these disadvantages, processes that involve the use as a foaming agent of water used as a dispersion medium have been proposed as processes for economically producing expanded polyolefin resin particles that can be suitably used for production of in-mold expanded molded products.
Proposed as a process that involves the use of water as a foaming agent is a process for producing expanded crystalline polyolefin polymer particles by: dispersing crystalline polyolefin polymer particles containing 10 to 70% by weight of an inorganic filler into water, which serves as a dispersion solution, in a closed vessel; impregnating the crystalline polyolefin polymer particles with the water, which serves as a dispersion medium, at a pressure of not lower than the saturated vapor pressure of the dispersion liquid and a temperature of not higher than the melting point of the crystalline polyolefin polymer particles, while keeping the dispersion liquid in a high-pressure zone under such temperature conditions that the crystallization of the polymer particles progresses; and then releasing the dispersion liquid into a low-pressure zone (e.g., Patent Literature 4). However, the expanded particles that are obtained by this process contain the inorganic filler in large amounts, and therefore are extremely small in cell diameter and tend to have a higher open cell ratio. As such, they are not sufficient in fusion, surface appearance, and mechanical properties such as compressive strength when processed into in-mold expanded molded products.
Further proposed is a process for producing expanded polyolefin resin particles by: dispersing polyolefin resin particles containing either an aqueous inorganic substance or a hydrophilic polymer into water in a closed vessel; heating the resin particles up to or above the softening temperature of the resin particles to turn the resin particles into hydrous polyolefin resin particles; and then releasing the dispersion liquid into a low-pressure zone (e.g., Patent Literatures 5 to 10). According to the descriptions, this process makes it possible to obtain expanded polypropylene resin particles with a high expansion ratio under low pressure in the vessel while using water, carbon dioxide, nitrogen, or the like, which are environmentally friendly, as a foaming agent.
However, in the case of such polyolefin resin particles containing a water-soluble inorganic substance as described in Patent Literature 6, an increase in amount of addition of the water-soluble inorganic substance for the purpose of increasing the expansion ratio or, in particular, making the expansion ratio eight times or more as high tends to result accordingly in extremely smaller cells. In consequence, in-mold expanded molded products made from the resultant expanded particles suffer from such problems as lower fusibility between expanded particles, thus bringing about degradation in commercial value of the molded products and deterioration in the rate at which the molded products are produced. Further, since the expansion ratio and the cell diameter change in conjunction with each other, there are problems of difficulties, for example, with adjusting (controlling) only the expansion ratio with the cell diameter held constant, which imposes limitations on production of expanded particles having desired properties.
Expanded polypropylene resin particles produced with use of carbon dioxide with the moisture content of such polypropylene resin particles containing a hydrophilic polymer as described in Patent Literature 7 held at 8% by weight or higher suffer from the disadvantage of tending to shrink immediately after foaming due to their high moisture content.
Furthermore, although the processes of Patent Literature 5 to 7 for producing expanded polyolefin resin particles containing a hydrophilic polymer make it possible to obtain expanded polyolefin resin particles with a high expansion ratio under low pressure in the vessel while using environment-friendly water as a foaming agent, cells in the resulting expanded particles tend to be extremely small or nonuniform. In-mold, expanded molded products made from the resultant expanded particles with a low expansion ratio have no particular problems. However, those made with a high expansion ratio under such molding conditions as a short molding cycle and a short curing time after molding in recent pursuit of lower production costs have such problems as wrinkles on their surfaces and distortion in their shapes due to their large dimensional shrinkage, thus bringing about degradation in their commercial value and deterioration in the rate at which they are produced.
Such polyolefin resin particles containing a hydrophilic polymer have the hydrophilic polymer added so that water is used as a foaming agent. However, because in general a hydrophilic polymer is poor in dispersibility in polyolefin resin particles, there are such disadvantages as occurrence of variations in expansion ratio of the expanded particles and proneness to poor fusion between expanded particles processed into in-mold expanded molded products.
Meanwhile, there is disclosed a process for producing expanded particles without extremely smaller cells by introducing carbon dioxide as a foaming agent into polymer particles containing a polypropylene glycol-polyethylene glycol polymer together with an inorganic substance (e.g., Patent Literature 11). Due to the low compatibility of the polypropylene glycol-polyethylene glycol polymer with polypropylene resin, the process tends to cause troubles such as occurrence of strand breakage due to poor dispersion in the step of preparing the polymer particles and, fluctuation in the feed of a molten resin in an extruder. Therefore, the polypropylene glycol-polyethylene glycol polymer can only be added in a minute amount, and due to its low water absorption rate, there has been no choice but to resort to foaming by carbon dioxide. Further, because the polypropylene glycol-polyethylene glycol polymer used has a great average molecular weight, it has been necessary to use polypropylene resin having a melt index of not less than 10 g/10 min, so as to obtain expanded polypropylene resin particles having a high expansion ratio. Furthermore, there have been such disadvantages as proneness to a lower rate of fusion between expanded particles processed into molded products, degradation in heat resistance, and degradation in strength.